Ink jet printers are popular and common in the computer field. These printers are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. Ink jet printers produce high-quality print, are compact and portable, and print quickly and quietly because only ink strikes a print medium (such as paper).
An ink jet printer produces a printed image by printing a pattern of individual dots (or pixels) at specific defined locations of an array. These dot locations, which are conveniently visualized as being small dots in a rectilinear array, are defined by the pattern being printed. The printing operation, therefore, can be pictured as the filling of a pattern of dot locations with dots of ink.
Ink jet printers print dots by ejecting a small volume of ink onto the print medium. These small ink drops are positioned on the print medium by a moving carriage that supports a printhead cartridge containing ink-drop generators. The carriage traverses over the print medium surface and positions the printhead cartridge depending on the pattern being printed. An ink supply, such as an ink reservoir, supplies ink to the drop generators. The drop generators are controlled by a microprocessor or other controller and eject ink drops at appropriate times upon command by the microprocessor. The timing of ink drop ejections typically corresponds to the pixel pattern of the image being printed.
In general, the drop generators eject ink drops through a nozzle or an orifice by rapidly heating a small volume of ink located within a vaporization or firing chamber. The vaporization of the ink drops typically is accomplished using an electric heater, such as a small thin-film (or firing) resistor. Ejection of an ink drop is achieved by passing an electric current through a selected firing resistor to superheat a thin layer of ink located within a selected firing chamber. This superheating causes an explosive vaporization of the thin layer of ink and an ink drop ejection through an associated nozzle of the printhead.
The resolution of an ink jet printer is directly related to the size and number of ink drops printed on a print medium. For example, for a given area a small number of large ink drops produces a relatively low-resolution printed image while a large number of small ink drops generally produces a higher-resolution printed image. The quality and resolution of printed images that a printer is capable of producing are often compared to photographs, and "photographic-quality" resolution means that the resolution approaches that of a photograph.
There is a continually increasing demand for low-cost ink jet printers that are capable of producing "photographic-quality" images. Achieving this high resolution while keeping costs low requires a careful balance between the architecture of the printhead (such as the architecture of the firing chamber, the firing resistor and the firing frequency) and the composition of liquid ink. Typically, a change in the printhead architecture or in the ink composition to solve one problem may create other problems. Thus, in order to produce an inexpensive ink jet printer capable of photographic-quality resolution several factors in the printhead architecture and ink composition should be taken into account.
For example, in order to produce photographic-quality resolution, an ink jet printer typically will use a printhead architecture that increases the number of drops per area (the dot resolution). This increased dot resolution generally is accomplished by greatly decreasing the size of each drop. However, one disadvantage of decreasing the drop size is that the speed of the printer is decreased because the area of print media covered by a single ink drop is also decreased.
Although this reduction in printer speed may be mitigated by increasing the number of drop generators in a given area (drop generator density) on a printhead, this creates other problems. In particular, an increase in the drop generator density means that the drop size must be reduced and the frequency of drops ejected (the firing frequency) increased. However, this reduction of drop size and increase in firing frequency creates a drop instability whereby the velocity and the volume of each drop has the undesirable characteristic of varying or decreasing over time. A decrease of drop velocity over time (during a single firing burst) is known as decel. Decel and other changes in drop velocity are undesirable because the accuracy of the drop placement on the print media is adversely affected.
As such, producing a low-cost ink jet printer capable of generating high-resolution printed output usually requires a careful balance between several factors. A change in printhead architecture that solves one problem may create other problems. Therefore, there exists a need for a low-cost ink jet printing system that provides high-resolution (such as photographic quality) output with a suitable printhead architecture and ink composition.